In image detection and analysis, it is desired to determine the frequency or wavelength spectrum of the light in addition to the total intensity from the image at different points on the image. In a first method, the light from an image is transmitted to a plurality of imagers (cameras). Each imager has a filter at a wavelength passband offset from the passband of the other filters and a detector for measuring the intensity of the filtered light. This method provides both simultaneous spatial and time wavelength distributions of the image. Although this method is simple, it is also cumbersome requiring many imagers and filters and it is not always possible to use it. For example, filters may not be available for some spectral regions. As another example, where filters are available, the filters may be limited in their transmissive or spectral discrimination properties.
In a second method, the light from the image is successively transmitted through a sequence of filters to a single imager. As with the first method, each filter has a passband offset from the passband of the other filters. Such a method provides a simultaneous spatial distribution of the image at the wavelength of the passband of each filter and a time sequence of the wavelength distribution by changing the filters. Unlike the first method, this method does not provide a simultaneous wavelength distribution of the image. Moreover, the system has dead time between measurements while the filters are being changed. Another disadvantage is that the rapid changing of filters requires a complex mechanical system, such as a filter wheel.
In a third method, a spectrometer has an entrance slit for passing a portion of an image, which is then dispersed into a spectrum by a grating or the like. An exit slit passes a portion of the dispersed light, which covers a narrow range of wavelengths, onto a detector for measuring the intensity of the light of this wavelength range. This method requires that the exit mask be moved relative to the detector to obtain a wavelength distribution of the image and that the spectrometer be moved relative to the image to obtain a spatial distribution of the image. For high spectral resolution, the spectrometer uses narrow entrance slits. Because the slits are small apertures, they limit the amount of light received at a detector. In other words, the light throughput, which is the product of the area of the input aperture multiplied by the angular divergence of the light rays being collected by the spectrometer, is strictly limited. Consequently, the noise of the detectors limits the measurable intensity of the dispersed light.